1. Field of the Invention
This invention relates, generally, to the art of filtration. More particularly, it relates to an apparatus that filters and separates solids and organic pollutants from solution.
2. Description of the Prior Art
Oil and solids must be separated from water as a preliminary step in both waste water treatment and oil recovery processes to facilitate further filtration and clarification.
Moreover, the need to effectively filter culture water that maintains aquatic organisms has increased due to the enactment of legislation that limits the use of municipal and ground water supplies.
Existing technologies for separation of mixed fluids includes a wide variety of complex machines such as deep hydroclones, centrifuges, and large circular tanks with bottom scrapers and surface skimmers. The fluid flow in these machines follows a spiral or rising path of travel. Other designs create horizontal flow over laminar plates in rectangular tanks.
After an initial separation step, the fluid being treated is filtered to remove impurities of both particulate and dissolved matter. Typically, a mechanical means having some type of matter-sifting screen is employed. Such techniques are effective but require expensive moving parts (such as a rotating screen, e.g.) and a backflow of fluid is needed to clean (backwash) the screen.
Particle filters use a preselected type of unconsolidated material such as sand or gravel to trap downwardly-flowing particulate matter in the upper layers at the surface of the media. The upper layers of the media become clogged easily; accordingly, frequent backwashing is required. Backwashing should be reduced to a minimum, however, because it causes loss of water and medium.
Gravitational or sedimentation processes simply hold water or slow the flow rate to a very low speed so that materials kept in suspension by moving water will eventually settle. To be effective, this type of filtration requires large containment areas to prolong the detention or dwell time of the fluid being treated.
Centrifuges and hydroclones, respectively, magnify the weight of suspended particles or heavier fluids by spinning the container or circulating the water flow. In applications requiring treatment of large volumes of fluid, such units can be prohibitively expensive or too large and deep, requiring excavation or high head pump pressure, depending upon site conditions.
A centrifuge, being mechanically driven, has additional maintenance costs as well. Units of this type are designed to remove grit and solids but fail to satisfactorily remove oily or greasy waste.
Filtration of waste water or recirculated water used to culture aquatic organisms must physically remove waste solids and remove or neutralize dissolved nutrients. The physical removal of waste solids is typically accomplished by employing a settling tank, a hydroclone or a straining type of filter as mentioned above.
Nitrification is the biological conversion of toxic ammonia waste compounds to harmless nitrates. Nitrification or biological filtration systems provide a medium with a large surface area for aerobic (oxygen consuming) bacteria to grow. This type of filtration is critical for the culture of aquatic organisms using recirculated water.
Biological contactors for biological filtration systems include submerged bed, trickling filters, rotating disks and drums, and fluidized bed types.
The submerged bed (commonly called undergravel) filtration system constrains culture water to flow vertically downward through a support media; the bacteria colonizes the support media. This is the least desirable nitrification process because the media is oxygen-limited and traps solids and bacterial waste products. Thus, the media becomes clogged and channeled, thereby leading to colonization of anaerobic (non-oxygen consuming) bacteria in the clogged or low flow areas known as dead spots. Anaerobic bacteria live only in the absence of oxygen and convert nitrate back to toxic nitrite and also produce hydrogen sulfide which is also very toxic to aquatic animals.
Trickling filters are the most popular nitrifiers for urban waste water and salt water aquaria applications. These filters are never oxygen limited because the substrate is suspended above the water with the flow trickling down. However, trickling filters do not capture solids and may even contribute to solids generation. The trickling filter is also subject to drying out and subsequent die-off of organisms if the flow is not constant. Accordingly, trickling filters require some means of solids separation both before and after the filtration unit.
Rotating discs and drums work by moving colonized media through waste water with intermittent contact with ambient air. These filters, like trickling filters, are therefore never oxygen limited. However, rotating media of this sort involves moving mechanical parts and therefore inherent power and wear problems. Filters of this type also do not capture solids.
Fluidized bed filters work by suspending a medium with a very large surface area and having a density slightly greater than that of water. This suspension of medium is maintained by a vertically upward flow. Filters of this type are resistant to clogging and are considered to be the most effective filters for aquaculture nitrification applications. However, problems with upflow suspended medium arise because such filters are oxygen limited and do not separate solids.
Some new unconsolidated bed filters use a floating medium in an upflow configuration. However, floating low density plastic pellets have been used for biological filtration in aquaculture applications since the 1950's. Recent innovations include means for pumping culture water upwardly through a floating "bed" of pellets; a strainer-type screen prevents the pellets from exiting the filter vessel. Problems with current applications of floating pellet filtration stem from the inherent pressure needed to push water up through the medium and the fast buildup of flocculent matter which further impedes water flow and causes pressure buildup. These filters are also oxygen-limited and dependent upon the oxygen content of the water coming into the filter. However, in view of the art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in this art how the needed improvements could be provided.